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      <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Semantic Actions</b></font>
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<p>Semantic actions have the form: <b>expression[action]</b></p>
<p>Ultimately, after having defined our grammar and having generated a corresponding 
  parser, we will need to produce some output and do some work besides syntax 
  analysis; unless, of course, what we want is merely to check for the conformance 
  of an input with our grammar, which is very seldom the case. Semantic actions 
  may be attached to any expression at any level within the parser hierarchy. 
  An action is a C/C++ function or function object that will be called if a match 
  is found in the particular context where it is attached. The action function 
  serves as a hook into the parser and may be used to, for example:</p>
<blockquote>
  <p><img src="theme/bullet.gif" width="13" height="13"> Generate output from 
    the parser (ASTs, for example)<br>
    <img src="theme/bullet.gif" width="13" height="13"> Report warnings or errors<br>
    <img src="theme/bullet.gif" width="13" height="13"> Manage symbol tables</p>
</blockquote>
<h2>Generic Semantic Actions (Transduction Interface)</h2>
<p>A generic semantic action can be any free function or function object that
  is compatible with the interface:</p>
<pre><code><font color="#000000"><span class=identifier></span><span class=keyword>    void </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>IteratorT </span><span class=identifier>first</span><span class=special>, </span><span class=identifier>IteratorT </span><span class=identifier>last</span><span class=special>);</span></font></code></pre>
<p>where <tt>IteratorT</tt> is the type of iterator used, <tt>first</tt> points 
  to the current input and <tt>last</tt> points to one after the end of the input 
  (identical to STL iterator ranges). A function object (functor) should have 
  a member <tt>operator()</tt> with the same signature as above:</p>
<pre><code><font color="#000000"><span class=special>    </span><span class=keyword>struct </span><span class=identifier>my_functor
    </span><span class=special>{
        </span><span class=keyword>void </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>IteratorT </span><span class=identifier>first</span><span class=special>, </span><span class=identifier>IteratorT </span><span class=identifier>last</span><span class=special>) </span><span class=keyword>const</span><span class=special>;
    </span><span class=special>};</span></font></code></pre>
<p>Iterators pointing to the matching portion of the input are passed into the 
  function/functor.</p>
<p>In general, semantic actions accept the first-last iterator pair. This is the 
  transduction interface. The action functions or functors receive the unprocessed 
  data representing the matching production directly from the input. In many cases, 
  this is sufficient. Examples are source to source translation, pre-processing, 
  etc. </p>
<h3>Example:</h3>
<pre><code><font color="#000000"><span class=special>    </span><span class=keyword>void
    </span><span class=identifier>my_action</span><span class=special>(</span><span class=keyword>char const</span><span class=special>* </span><span class=identifier>first</span><span class=special>, </span><span class=keyword>char const</span><span class=special>* </span><span class=identifier>last</span><span class=special>)
    {
        </span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special> </span><span class="identifier">str</span><span class=special>(</span><span class=identifier>first</span><span class=special>, </span><span class=identifier>last</span><span class=special>);
        </span><span class=identifier>std</span><span class=special>::</span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=identifier>str </span><span class=special>&lt;&lt; </span><span class=identifier>std</span><span class=special>::</span><span class=identifier>endl</span><span class=special>;
    }

    </span><span class=identifier>rule</span><span class=special>&lt;&gt; </span><span class=identifier>myrule </span><span class=special>= (</span><span class=identifier>a </span><span class=special>| </span><span class=identifier>b </span><span class=special>| *(</span><span class=identifier>c </span><span class=special>&gt;&gt; </span><span class=identifier>d</span><span class=special>))[&</span><span class=identifier>my_action</span><span class=special>];</span></font></code></pre>
<p>The function <tt>my_action</tt> will be called whenever the expression <tt>(a 
  | b | *(c &gt;&gt; d)</tt> matches a portion of the input stream while parsing. 
  Two iterators, <tt>first</tt> and <tt>last</tt>, are passed into the function. 
  These iterators point to the start and end, respectively, of the portion of 
  input stream where the match is found.</p>
<h3>Const-ness:</h3>
<p>With functors, take note that the <tt>operator()</tt> should be <tt>const</tt>.
  This implies that functors are immutable. One may wish to have some member variables
  that are modified when the action gets called. This is not a good idea. First
  of all, functors are preferably lightweight. Functors are passed around a lot
  and it would incur a lot of overhead if the functors are heavily laden. Second,
  functors are passed by value. Thus, the actual functor object that finally attaches
  to the parser, will surely not be the original instance supplied by the client.
  What this means is that changes to a functor's state will not affect the original
  functor that the client passed in since they are distinct copies. If a functor
  needs to update some state variables, which is often the case, it is better
  to use references to external data. The following example shows how this can
  be done:</p>
<pre><code><font color="#000000"><span class=special>    </span><span class=keyword>struct </span><span class=identifier>my_functor
    </span><span class=special>{
        </span><span class=identifier>my_functor</span><span class=special>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>& </span><span class=identifier>str_</span><span class=special>)
        </span><span class=special>: </span><span class=identifier>str</span><span class=special>(</span><span class=identifier>str_</span><span class=special>) </span><span class=special>{}

        </span><span class=keyword>void
        </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>IteratorT </span><span class=identifier>first</span><span class=special>, </span><span class=identifier>IteratorT </span><span class=identifier>last</span><span class=special>) </span><span class=keyword>const
        </span><span class=special>{
            </span><span class=identifier>str</span><span class=special>.</span><span class=identifier>assign</span><span class=special>(</span><span class=identifier>first</span><span class=special>, </span><span class=identifier>last</span><span class=special>);
        </span><span class=special>}

        </span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>& </span><span class=identifier>str</span><span class=special>;
    </span><span class=special>};</span></font></code></pre>
<h3>Full Example:</h3>
<p>Here now is our calculator enhanced with semantic actions:</p>
<pre><code><font color="#000000"><span class=special>    </span><span class=keyword>namespace
    </span><span class=special>{
        </span><span class=keyword>void    </span><span class=identifier>do_int</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>str</span><span class=special>, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>end</span><span class=special>)
        </span><span class=special>{
            </span><span class=identifier>string  </span><span class=identifier>s</span><span class=special>(</span><span class=identifier>str</span><span class=special>, </span><span class=identifier>end</span><span class=special>);
            </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=string>"PUSH(" </span><span class=special>&lt;&lt; </span><span class=identifier>s </span><span class=special>&lt;&lt; </span><span class=literal>')' </span><span class=special>&lt;&lt; </span><span class=identifier>endl</span><span class=special>;
        </span><span class=special>}

        </span><span class=keyword>void    </span><span class=identifier>do_add</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*)    </span><span class=special>{ </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=string>"ADD\n"</span><span class=special>; </span><span class=special>}
        </span><span class=keyword>void    </span><span class=identifier>do_subt</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*)   </span><span class=special>{ </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=string>"SUBTRACT\n"</span><span class=special>; </span><span class=special>}
        </span><span class=keyword>void    </span><span class=identifier>do_mult</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*)   </span><span class=special>{ </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=string>"MULTIPLY\n"</span><span class=special>; </span><span class=special>}
        </span><span class=keyword>void    </span><span class=identifier>do_div</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*)    </span><span class=special>{ </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=string>"DIVIDE\n"</span><span class=special>; </span><span class=special>}
        </span><span class=keyword>void    </span><span class=identifier>do_neg</span><span class=special>(</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*)    </span><span class=special>{ </span><span class=identifier>cout </span><span class=special>&lt;&lt; </span><span class=string>"NEGATE\n"</span><span class=special>; </span><span class=special>}
    </span><span class=special>}</span></font></code></pre>
<p>We augment our grammar with semantic actions:</p>
<pre><code><font color="#000000"><span class=special>    </span><span class=keyword>struct </span><span class=identifier>calculator </span><span class=special>: </span><span class=keyword>public </span><span class=identifier>grammar</span><span class=special>&lt;</span><span class=identifier>calculator</span><span class=special>&gt;
    </span><span class=special>{
        </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
        </span><span class=keyword>struct </span><span class=identifier>definition
        </span><span class=special>{
            </span><span class=identifier>definition</span><span class=special>(</span><span class=identifier>calculator </span><span class=keyword>const</span><span class=special>& </span><span class=identifier>self</span><span class=special>)
            </span><span class=special>{
                </span><span class=identifier>expression
                    </span><span class=special>=   </span><span class=identifier>term
                        </span><span class=special>&gt;&gt; </span><span class=special>*(   </span><span class=special>(</span><span class=literal>'+' </span><span class=special>&gt;&gt; </span><span class=identifier>term</span><span class=special>)[&</span><span class=identifier>do_add</span><span class=special>]
                            </span><span class=special>|   </span><span class=special>(</span><span class=literal>'-' </span><span class=special>&gt;&gt; </span><span class=identifier>term</span><span class=special>)[&</span><span class=identifier>do_subt</span><span class=special>]
                            </span><span class=special>)
                    </span><span class=special>;

                </span><span class=identifier>term </span><span class=special>=
                    </span><span class=identifier>factor
                        </span><span class=special>&gt;&gt; </span><span class=special>*(   </span><span class=special>(</span><span class=literal>'*' </span><span class=special>&gt;&gt; </span><span class=identifier>factor</span><span class=special>)[&</span><span class=identifier>do_mult</span><span class=special>]
                            </span><span class=special>|   </span><span class=special>(</span><span class=literal>'/' </span><span class=special>&gt;&gt; </span><span class=identifier>factor</span><span class=special>)[&</span><span class=identifier>do_div</span><span class=special>]
                            </span><span class=special>)
                        </span><span class=special>;

                </span><span class=identifier>factor
                    </span><span class=special>=   </span><span class=identifier>lexeme_d</span><span class=special>[(+</span><span class=identifier>digit_p</span><span class=special>)[&</span><span class=identifier>do_int</span><span class=special>]]
                    </span><span class=special>|   </span><span class=literal>'(' </span><span class=special>&gt;&gt; </span><span class=identifier>expression </span><span class=special>&gt;&gt; </span><span class=literal>')'
                    </span><span class=special>|   </span><span class=special>(</span><span class=literal>'-' </span><span class=special>&gt;&gt; </span><span class=identifier>factor</span><span class=special>)[&</span><span class=identifier>do_neg</span><span class=special>]
                    </span><span class=special>|   </span><span class=special>(</span><span class=literal>'+' </span><span class=special>&gt;&gt; </span><span class=identifier>factor</span><span class=special>)
                    </span><span class=special>;
            </span><span class=special>}

            </span><span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt; </span><span class=identifier>expression</span><span class=special>, </span><span class=identifier>term</span><span class=special>, </span><span class=identifier>factor</span><span class=special>;

            </span><span class=identifier>rule</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>&gt; </span><span class=keyword>const</span><span class=special>&
            </span><span class=identifier>start</span><span class=special>() </span><span class=keyword>const </span><span class=special>{ </span><span class=keyword>return </span><span class=identifier>expression</span><span class=special>; </span><span class=special>}
        </span><span class=special>};
    </span><span class=special>};</span></font></code></pre>
<p>Feeding in the expression <tt>(-1 + 2) * (3 + -4)</tt>, for example, to the
  rule <tt>expression</tt> will produce the expected output:</p>
<pre><code><span class=special>-</span><span class=number>1
</span><span class=number>2
</span><span class=identifier>ADD
</span><span class=number>3
</span><span class=special>-</span><span class=number>4
</span><span class=identifier>ADD
</span><span class=identifier>MULT</span></code></pre>
<p>which, by the way, is the Reverse Polish Notation (RPN) of the given expression, 
  reminiscent of some primitive calculators and the language Forth.</p>
<p><img src="theme/lens.gif" width="15" height="16"> <a href="../example/fundamental/calc_plain.cpp">View 
  the complete source code here</a>. This is part of the Spirit distribution. 
</p>
<h2><a name="specialized_actions"></a>Specialized Actions</h2>
<p>In general, semantic actions accept the first-last iterator pair. There are 
  situations though where we might want to pass data in its processed form. A 
  concrete example is the numeric parser. It is unwise to pass unprocessed data 
  to a semantic action attached to a numeric parser and just throw away what has 
  been parsed by the parser. We want to pass the actual parsed number.</p>
<p>The function and functor signature of a semantic action varies depending on 
  the parser where it is attached to. The following table lists the parsers that 
  accept unique signatures.</p>
<table width="80%" border="0" align="center">
  <tr> 
    <td class="note_box"><img src="theme/note.gif" width="16" height="16"> Unless 
      explicitly stated in the documentation of a specific parser type, parsers 
      not included in the list by default expect the generic signature as explained 
      above.</td>
  </tr>
</table>
<h3>Numeric Actions</h3>
<p><b>Applies to:</b></p>
<blockquote>
  <p><img src="theme/bullet.gif" width="13" height="13"> uint_p<br>
    <img src="theme/bullet.gif" width="13" height="13"> int_p<br>
    <img src="theme/bullet.gif" width="13" height="13"> ureal_p<br>
    <img src="theme/bullet.gif" width="13" height="13"> real_p</p>
</blockquote>
<p><b>Signature for functions:</b></p>
<pre><code><font color="#000000"><span class=identifier>    </span><span class=keyword>void </span><span class=identifier>func</span><span class=special>(</span><span class=identifier>NumT </span><span class=identifier>val</span><span class=special>);</span></font></code></pre>
<p><b>Signature for functors:</b> </p>
<pre><code><font color="#000000"><span class=special>    </span><span class=keyword>struct </span><span class=identifier>ftor
    </span><span class=special>{
        </span><span class=keyword>void </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>NumT </span><span class=identifier>val</span><span class=special>) </span><span class=keyword>const</span><span class=special>;
    </span><span class=special>};</span></font></code></pre>
<p>Where <tt>NumT</tt> is any primitive numeric type such as <tt>int</tt>, <tt>long</tt>,
  <tt>float</tt>, <tt>double</tt>, etc., or a user defined numeric type such as
  big_int. <tt>NumT</tt> is the same type used as template parameter to <tt>uint_p</tt>,
  <tt>int_p</tt>, <tt>ureal_p</tt> or <tt>real_p</tt>. The parsed number is passed
  into the function/functor.</p>
<h3>Character Actions</h3>
<p><b>Applies to:</b></p>
<blockquote>
  <p><img src="theme/bullet.gif" width="13" height="13"> chlit, ch_p<br>
    <img src="theme/bullet.gif" width="13" height="13"> range, range_p<br>
    <img src="theme/bullet.gif" width="13" height="13"> anychar<br>
    <img src="theme/bullet.gif" width="13" height="13"> alnum, alpha<br>
    <img src="theme/bullet.gif" width="13" height="13"> cntrl, digit<br>
    <img src="theme/bullet.gif" width="13" height="13"> graph, lower<br>
    <img src="theme/bullet.gif" width="13" height="13"> print, punct<br>
    <img src="theme/bullet.gif" width="13" height="13"> space, upper<br>
    <img src="theme/bullet.gif" width="13" height="13"> xdigit</p>
</blockquote>
<p><b>Signature for functions:</b></p>
<pre><code><font color="#000000"><span class=identifier>    </span><span class=keyword>void </span><span class=identifier>func</span><span class=special>(</span><span class=identifier>CharT </span><span class=identifier>ch</span><span class=special>);</span></font></code></pre>
<p><b>Signature for functors:</b></p>
<pre><code><font color="#000000"><span class=special>    </span><span class=keyword>struct </span><span class=identifier>ftor
    </span><span class=special>{
        </span><span class=keyword>void </span><span class=keyword>operator</span><span class=special>()(</span><span class=identifier>CharT </span><span class=identifier>ch</span><span class=special>) </span><span class=keyword>const</span><span class=special>;
    </span><span class=special>};</span></font></code></pre>
<p>Where <tt>CharT</tt> is the value_type of the iterator used in parsing. A <tt>char
  const*</tt> iterator for example has a <tt>value_type</tt> of <tt>char</tt>.
  The matching character is passed into the function/functor.</p>
<h2>Cascading Actions</h2>
<p>Actions can be cascaded. Cascaded actions also inherit the function/functor
  interface of the original. For example:</p>
<pre><code><font color="#000000"><span class=special>    </span><span class=identifier>uint_p</span><span class=special>[</span><span class=identifier>fa</span><span class=special>][</span><span class=identifier>fb</span><span class=special>][</span><span class=identifier>fc</span><span class=special>]</span></font></code></pre>
<p>Here, the functors <tt>fa</tt>, <tt>fb</tt> and <tt>fc</tt> all expect the
  signature <tt>void operator()(unsigned n) const</tt>.</p>
<h2>Directives and Actions</h2>
<p>Directives inherit the function/functor interface of the subject it is
  enclosing. Example:</p>
<pre><code><font color="#000000"><span class=special>    </span><span class=identifier>as_lower_d</span><span class=special>[</span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'x'</span><span class=special>)][</span><span class=identifier>f</span><span class=special>]</span></font></code></pre>
<p>Here, the functor <tt>f</tt> expects the signature <tt>void operator()(char 
  ch) const</tt>, assuming that the iterator used is a <tt>char const*</tt>.</p>
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